Conventionally, a metallic pipe for hot water circulation is used primarily in floor heating by the hot water circulation method. When laying such a pipe for hot water circulation, the pipe is embedded inside concrete and installed under the floor in most cases, and the maintenance and repair becomes extremely difficult once the pipe has been installed. Furthermore, the pipe is normally required to have a long-term durability of about 50 years. Based on such severe conditions, a use of a plastic pipe that is cheaper than a metallic pipe and also does not involve corrosion of the pipe material is desired. Polyethylene, polypropylene, polybutene, etc., are used as the plastic pipe materials.
However, when such a plastic pipe is used in a floor heating system based on the hot water circulation method, there is a problem of corrosion of the metallic parts of heat exchanger, pump and the like, due to oxygen. A possible reason of this corrosion is that the oxygen present in the atmosphere passes through the plastic walls, and penetrates and dissolves in the hot water circulating within the pipe. Thus, a multilayer polyethylene pipe having aluminum as the intermediate layer is used, but due to cracking of the aluminum layer possibly due to the change in temperature, this multilayered pipe cannot prevent corrosion due to the oxygen.
As a solution to this problem, various multilayered pipes including a plastic resin having excellent oxygen gas barrier property and polyethylene are evaluated. Among these, a multilayered pipe using an ethylene-vinyl alcohol copolymer (hereinafter, may be abbreviated as EVOH) is confirmed to have the highest oxygen barrier property and most excellent mechanical strength, and these days, EVOH multilayer plastic pipes are used widely as pipes for hot water circulation. For example, a hot water circulation pipe including an EVOH layer as the intermediate layer (see Patent document 1: JP 61-140691 A and a hot water circulation pipe having an EVOH layer as the outermost layer (see Patent document 2: JP 02-74341 A are known.
However, in the case of a hot water circulation pipe, the exposure of the EVOH layer to remarkably high temperatures for a long period of time gradually accelerates oxidation degradation of EVOH due to the oxygen present in the air. Although the oxygen barrier property of EVOH is not seriously affected by a small amount of degradation, the mechanical strength may decline due to oxidation degradation giving rise to cracks parallel to the lengthwise direction of the EVOH layer. When cracking thus occurs as a result of aging degradation, the excellent oxygen barrier property of EVOH is obviously lost, and with these cracks as the starting point, fissures running through the pipe walls might occur. In such a case, the water in the pipe leaks out under the floor causing a big problem.
The addition of an antioxidant is effective against the oxidation degradation of EVOH in the air, and a composition containing EVOH and an amide having a hindered phenol group (see Patent document 3: JP 63-286459 A) as well as a composition in which an aliphatic carboxylic acid containing three to nine carbon atoms and a hindered phenol antioxidant are mixed with EVOH (see Patent document 4: JP 4-227744 A) are known. By making use of these techniques, it becomes possible to extend the life of the hot water circulation pipe to a large extent.
However, in the recent resource-saving and energy-saving trend, where a large increase in the lifespan of household equipment is demanded, the lifespan of the hot water circulation pipes is also desired to be much more.
Furthermore, until now, floor heating by the hot water circulation method is mainly prevalent in western countries such as Northern Europe, but in the recent years, this heating system is gradually beginning to gain popularity in Japan as well. In Japanese houses, there are many restrictions in the floor area for laying down hot water circulation pipes, and in many cases the pipes have to be installed with a small bending radius. Therefore, when secondary processing is performed to bend the pipe, cracking occurs easily in a direction perpendicular to the lengthwise direction of the EVOH layer. Further improvements are necessary even with regard to cracking due to the distortion during such secondary processing.
Furthermore, an insulated multilayered pipe including an inner pipe carrying gas or liquid, an insulation foam layer covering the inner pipe, and an outer pipe covering the insulation foam layer is used as a pipe for district energy (area-wide air conditioning) (see Patent document 5: EP 0634602 A1). Conventionally, the insulation foam layer is surrounded by a foil (film) adhering on to the outer pipe. Examples of the foil include metals such as aluminum, and polyethylene from the viewpoint of a thermoformable material. However, the conventionally used aluminum foil is difficult to handle, and on the other hand, the polyethylene film fails to provide long-term insulation effect without proper permeation prevention of carbon dioxide, pentane, and cyclopentane used in the blowing agent of the insulation foam. Therefore, the use of a multilayered pipe including EVOH having gas barrier property as an outer pipe is being examined from the viewpoint of improvement in handling and permeation prevention of carbon dioxide, pentane, and cyclopentane (see Patent document 6: EP 1355103 A1). However, even the insulated multilayered pipe has a problem of acceleration of oxidation degradation of EVOH due to the oxygen in the air during long-term usage. When the mechanical strength declines and cracks occur parallel to the lengthwise direction of the EVOH layer as a result of oxidation degradation, blowing gas is dispersed from the insulated multilayered pipe and the insulation performance declines. In order to solve such a problem, the EVOH layer is desired to have a long life.
On the other hand, it is disclosed that by using an EVOH resin composition (not a blended composition with polyolefin) containing a conjugated polyene compound having a boiling point of 20° C. or more in EVOH, a molded article with less coloring and less formation of a gel-type object during molding is obtained (see Patent document 7: JP 9-71620 A), but the patent document does not examine the decline in the mechanical strength due to oxidation degradation at high temperatures.